Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

[Dave Hansen <dave.hansen@intel.com>]

On 6/10/19 4:20 PM, H.J. Lu wrote:
>>> Perhaps we still let the app fill the bitmap?
>> I think I'd want to see some performance data on it first.
> Updating legacy bitmap in user space from kernel requires
> 
> long q;
> 
> get_user(q, ...);
> q |= mask;
> put_user(q, ...);
> 
> instead of
> 
> *p |= mask;
> 
> get_user + put_user was quite slow when we tried before.

Numbers, please.

There are *lots* of ways to speed something like that up if you have
actual issues with it.  For instance, you can skip the get_user() for
whole bytes.  You can write bits with 0's for unallocated address space.
 You can do user_access_begin/end() to avoid bunches of STAC/CLACs...

The list goes on and on. :)